Various glass compositions are disclosed for the glass composition for a substrate used in displays according to the type of the displays. A plasma display panel (hereinafter referred to as "PDP" for brevity) has been attracting attention as a wide-area display for wall type high-definition direction vision TV sets. Soda-lime glass conventionally used in the field of construction has been used as a glass substrate of PDP. In the production process of PDP, the glass substrate is subjected to heat treatment at 500 to 600.degree. C. in steps of baking of electrodes to the substrate, formation of a dielectric layer, formation of a partition wall, formation of a fluorescent substance, and the like. Thermal shrinkage of the glass substrate generated in these heat treatments results in position shift in matching a pattern or laminating a surface glass sheet and a back glass sheet in the subsequent step. Therefore, in producing a large-sized or high-definition PDP, it is necessary to control the thermal expansion coefficient of the glass substrate and its variation. Further, because the glass substrate is combined with an insulating paste, sealing frits, etc., in the above heat treatments, the glass substrate is also required to match with these materials in coefficient of thermal expansion. Accordingly, it is necessary for the glass substrate to have an average thermal expansion coefficient of about 75.times.10.sup.-7 to 100.times.10.sup.-7 /K in a temperature range of from 50 to 350.degree. C.
In order to decrease thermal shrinkage of glass in the heat treatments, it is necessary to use glass having a high strain point. A non-alkali glass substrate used in liquid crystal displays has a high strain point but is unsuitable for use in PDP because of its small thermal expansion coefficient. On the other hand, the conventional soda-lime glass, which has been produced by a float method that can produce large-area glass plates inexpensively, satisfies the requirement of thermal expansion coefficient for use in PDP. However, such a glass has a strain point of about 500.degree. C., and considerable shrinkage occurs when it is subjected to the heat treatments at 500 to 600.degree. C. Therefore, the conventional soda-lime glass is not suitable for use in large-sized or high-definition PDP.
Means for solving the above-described problem of thermal shrinkage are disclosed in, for example, JP-A-3-40933, JP-A-7-257937,and JP-A-8-165138 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, since the glass compositions disclosed in the Examples of these Japanese publications contain large amounts of Al.sub.2 O.sub.3 and/or ZrO.sub.2, the glass tends to cause devitrification due to the alumina or zirconia content of liner bricks of a furnace when a glass is produced using a general melting furnace. Thus, there is a problem on the quality of glass.
JP-A-8-133778 and WO 96/11887 disclose glass compositions suitable for use in PDP, in which Al.sub.2 O.sub.3 and ZrO.sub.2 are not essential. However, WO 96/11887 demonstrates in its Examples a glass composition having a large amount of Al.sub.2 O.sub.3 and/or ZrO.sub.2, showing the above-described disadvantage. Further, JP-A-8-133778 demonstrates in its Examples glass compositions having an Al.sub.2 O.sub.3 content of 5.6% by weight or more, which involve the same problem of devitrification in a furnace as described above or the problem of difficulty in melting due to its melting temperature (a temperature at which the composition has a viscosity of 10.sup.2 poise) as high as 1,564.degree. C. or higher.